Refractometer Use Techniques And Refraction Principles; High Index/High Dispersion Samples; Calibration With Test Glass - Reichert Abbe Mark II Manual De Instrucciones

8.0 REFRACTOMETER USE TECHNIQUES AND
REFRACTION PRINCIPLES
Refractive Index measurements taken on the REICHERT ABBE
MARK II and REICHERT ABBE MARK II Plus Refractometers
are affected by temperature. Different values can be obtained
when the same sample is measured at different temperatures.
For example: as the temperature of a sugar solution sample rises,
its density and refractive index decrease. This causes an apparent
decrease in concentration when using the non-temperature
compensated % Solids mode. The % Solids-TC (Temperature
Compensated) mode automatically corrects the reading to the
correct value at 20°C.
For every 1°C change in temperature, an aqueous solution
containing sugar will change readings by approximately 0.08%.
This is the average value for samples of 0 to 85% at 15°C to
45°C. As temperature rises the % Solids reading decreases. As
temperature decreases, the % Solids reading increases.
The REICHERT ABBE MARK II Refractometer temperature will
rise approximately 1°C every 10 minutes for 1 hour until it stabilizes.
An instrument that has been turned on for more than 1 hour in a
20°C room could read 26°C. The rise in temperature is principally
due to heat generated by the power transformer inside the
instrument housing.
The sample placed on the measuring prism surface of a
REICHERT ABBE MARK II Refractometer that has been turned
on for more than an hour in a 20°C environment will assume the
temperature of the prism (i.e., approximately 26°C). Although the
sample is now warmer and would indicate a lower concentration
when measured in the uncompensated Brix mode, the BX-TC
mode automatically compensates for the temperature difference
and displays the concentration of the solution as if its temperature
were 20°C. Without compensation for the 6° rise, the readout of a
70% solution would be 0.5% less (6°C x 0.08%) or 69.5%.
The measuring prism surface temperature can be lowered by running
cooling fluid through the water jackets in the prism housing. However,
the heat generated inside the instrument housing still affects the
prism temperature. For this reason it is necessary to run fluid slightly
below the desired temperature. A thermometer placed in the fluid line
may measure several degrees below the actual displayed temperature
of the measuring prism surface displayed when the temperature button
is pressed. The opposite is true if the prism temperature has to be
elevated above the ambient temperature.
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The fluid circulation system allows the prism to be adjusted for
temperatures up to 80°C. This may be necessary when using
samples that solidify at lower temperatures.
Measuring viscous samples, such as high Brix sugar syrups,
may be more difficult than measuring products with lower total
dissolved solids.
Stir the sample on the measuring prism surface with a rubber
spatula or an angled piece of clear plastic tubing. Completely
cover the surface making sure all the air pockets between the
sample and the measuring prism surface are removed.
For removing carbonation from carbonated soft drink samples,
open and close the prism cover two or three times after loading.
Doing this provides the best possible reading line and the best
way to judge where the shadow line intersects the crosshair reticle.
NOTE: Remember the accuracy of the instrument is only as good
as the clarity of the shadow line.

8.1 High Index/High Dispersion Samples

High refractive index samples with high dispersion properties
may require additional use of Catalog #13K30650 589nm
bandpass filter accessory. This accessory will insure that the
shadowline will be sharp for these samples.
9.0 CALIBRATION WITH THE TEST GLASS
Refer to Chart 1, Refractive Index of Triple Distilled Water at Various
Temperatures to compare the refractive index reading with the
calibrated temperature reading.
To calibrate the REICHERT ABBE MARK II Refractometer use:
• a refractive index test glass (Catalog No. 13104510)
• a bottle of 1- Bromonaphthaline (Catalog No. 13104520)
• a 1/16" (No. 4) Allen wrench (Catalog No. 13X30600)
• lens tissue
The refractive index test glass is imprinted with the refractive
index scale. The bottom and front surfaces of the glass are
polished. It is important that these surfaces are very clean when
beginning calibration procedures.
To calibrate:
1. Turn the " Mode selector switch to the refractive index position.
2. Apply a single drop of 1-Bromonaphthaline to the surface of
the measuring prism surface.